Jelmer-
I recently wrote the How to: Work with conversations by using EWS in Exchange article on MSDN. The GetConversationItems EWS operation, which was introduced in Exchange 2013, is what you want to use to retrieve everything in a specific conversation. GetConversationItems uses the ConversationId element, which wasn't around in Exchange 2007. ConversationTopic and ConversationIndex were around in Exchange 2007 - and I would assume you could use another operation to find all items with a specific ConversationTopic, but I haven't had to do that. If you want help with that approach, let me know.
The ConversationThread is the value you identified above (Ac9IKUBx8yX3vSXYQSWcb8ggCfC7FQ), on the first message, then the ConversationIndex is incremented every time the item is responded to, forwarded, etc.
From the article: "Exchange applies the same ConversationTopic value to replies to the first message and then updates the ConversationIndex value to represent the message’s position relative to the original message. If the subject of the email thread changes, Exchange applies a new ConversationTopic value and new ConversationIndex values to the new conversation."
I noticed that what you referred to in your post as the EWS docs is actually the CDO docs. The EWS reference content for Exchange 2013 is located here: http://msdn.microsoft.com/en-us/library/bb204119(v=exchg.150).aspx. And the EWS reference content for Exchange 2007 (if you want to compare), is here: http://msdn.microsoft.com/en-us/library/bb204119(v=exchg.80).aspx.
Hope this helps and please mark the post as answered if I did indeed answer your question.
Thanks, Mimi
Updated on 3/27 to respond to additional questions about ConversationId:
I just played around with this to be sure and the ConversationId does change if the subject is changed, but it stays the same so long as the message is just forwarded and replied to. My original ConversationId was:
<ConversationId Id="AAQkADI2MDY4MjQzLWQ1YzQtNGZkNi1iZmE4LTRjZmZmNGEyZjQ0YwAQAJJCqNI7ZfhJmIh7j2kgJvk=" />
<ConversationTopic>Original</ConversationTopic>
but after changing the subject twice it looked like this:
<ConversationId Id="AAQkADI2MDY4MjQzLWQ1YzQtNGZkNi1iZmE4LTRjZmZmNGEyZjQ0YwAQAEjf5yU/W9VMo6HxuanvhYE=" />
<ConversationTopic>2nd subject</ConversationTopic>
So the first X characters are the same, but then there's divergence.